A new report on Monday stated that the United States Air Force did not submit Devin Patrick Kelley’s criminal history to the FBI as required by the Pentagon, which would have prevented him from purchasing firearms.

Kelley, 26, opened fire on a congregation of churchgoers on Sunday, killing at least 26 people in the deadliest mass shooting at a place of worship in American history.

Kelley, who used an "assault-style rifle" and owned two handguns, should not have been able to legally purchase the weapons due to his history of violent criminal behavior — but the government made a mistake, NPR reported. "Under federal law, his conviction disqualified him from legally possessing a firearm."

"He assaulted his stepson severely enough that he fractured his skull, and he also assaulted his wife," retired colonel Don Christensen told The New York Times. "He pled to intentionally doing it."

A Pentagon official told NPR that a mistake made by the government resulted in his criminal records not being entered into the National Instant Criminal Background Check System (NICS).

"This was mishandled by the Air Force Office of Special Investigations at Holloman Air Force Base in New Mexico, where Kelley was serving when he was arrested," NPR explained. "An investigation is now underway, and the Air Force is taking it very seriously, said the source."

The Associated Press confirmed the information in a tweet: